Signal system.



K. DEMBNT. SIGNAL SYSTEM.

APPLICATION FILED MAR. 17, 1911.

1 ,0073545. Patented Oct. 31, 1911.

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MBIA PLANOGRAPl-l cO., WASHINGTON. n. c.

A. K. DEMENT. SIGNAL SYSTEM.

APPLICATION FILED MAR. 17, 1911.

1,007,545. Patented 0011.31, 1911.

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WITNESSES INVENTOR W ahwwm;

COLUMBIA PLANOORAPH con vllxsuma'roN. n. c.

UNITED STATES PATENT QFFICE.

ALFRED K. DEIVIENT, OF-PITTSBURGH, PENNSYLVANIA, ASSIGNOR TO UNITED STATES POLICE AND FIRE SIGNAL COMPANY, OF PITTSBURGH, PENNSYLVANIA, A CORPO- RATION OF PENNSYLVANIA.

SIGNAL SYSTEM.

To all whom it may concern.-

Be it known that I, ALFRED K. DEMENT, a resident of Pittsburgh, in the county of Allegheny and State of Pennsylvania, have invented a new and useful Improvement in Signal Systems, of which the following is a full, clear, and exact description, reference being had to the accompanying drawings, forming part of this specification, in which- Figure 1 is a general diagram of a system embodying my invention and also illustrating the initial positions of the selectors at the several call boxes or stations; Fig. 2 is a detail wiring diagram showing the apparatus and circuits at central station, at one of the call boxes or stations, and at the end of the line; and Figs. 3, 4 and 5 are detail views of one form of selector which may be employed.

My invention has relation to signal systems, and is designed to provide a system which will enable communication to be held in either direction between a central station and a plurality of call stations.

The invention is applicable to police signal systems, railway systems, mine systems, manufactories, offices, etc, and in fact, to any system in which it is desired to provide means whereby any one of a plurality of stations may at any time communicate with a central station, and in which the central station may either send a selective or collective call to the call stations.

A more specific object of my invention is to provide a system of this kind which can be operated with any desired number of call stations by the use of three main circuit wires or conductors common to all the call stations.

My improved system comprises a twowire metallic circuit over the wires of which telephone and other messages can be sent from any of the call stations to the central station; a third wire having a signal or signals connected thereto at each call station and normally grounded at the power end of the system; means whereby by the use of a current other than the current normally flowing over the two-wire metallic circuit, each of the said Wires may be separately grounded at its far end and made available for use as a grounded circuit in the selective or collective operation of the system; and

Specification of Letters Patent.

Application filed March 17, 1911.

Patented Oct. 31, 1911.

Serial No. 615,146.

means whereby the ground connections of said Wires may be broken to restore the normal condition of the two-wire circuit after operation.

My invention also comprises selective means at each station operated by the sepa rately grounded circuits to establish circuit conditions at any particular call station or at all call stations, whereby a signal may be operated at such station or stations by current furnished over the third wire.

Various other features comprised in my invention Will be hereinafter described and pointed out.

Referring to the drawings, the letters A, B and C designate the three line wires, the wires B and O constituting a normal twowire metallic circuit.

D D are alternating current generators at the central station.

E is a central station telephone battery, and F is a central station signal battery normally grounded at one side.

My system is a central energy system, and the necessary currents for operating it are furnished by these batteries and generators. For convenience of illustration, I have shown two separate generators, but it will be understood that in practice the necessary generator connections can be made from a single generator or other source of alternating current.

G is a switch for controlling the generator D, and having the four relatively fixed contact points 1, 2, 3 and 4, and two movable contacts 5 and 6.

G is a switch for the generator D, and having the four contact points 7, 8, 9 and 10 and the two movable contacts 11 and 12.

H is a reversing key having the four contact points 13, 14, 15 and 16 and the two movable contacts 17 and 18.

I is a selector of the interrupting type and of any well known or desired character. I may, for instance, use an ordinary messenger call bOX'Wli-l'l its contacts modified to give the required current impulses.

J is a gang key having at one side the four contact points 19, 20, 21 and 22, and the movable contacts 27 and 28, and at the opposite side the four contact points 23, 24c, 25 and 26 and the two movable contacts 29 and 30.

K is a difierentially wound relay normally connected across the two-wire circuit B C and controlling the movable contact 71: of a local circuit, which includes the register or recorder L.

OP designates the central. station operators telephone transmitter which is bridged across the conductors B C through the operators switch OS.

()R is the operators receiver.

0 designates a telephone set at one of the call stations.

0 the call station telephone switch, P a code transmitter at the call station which may be of any well known or suitable type, Q a signal or call lamp at said station, and R a compound relay for controlling the circuit of the lamp.

Each of the line wires B and C at the far end of the system is grounded through a condenser S or S, and also has a normally open shunt connection T or T to ground, controlled by a relay U or U, and the line wire A is also grounded at the far end of the system.

In the diagram shown in Fig. 2, all the switches are in their normal positions. The switch G controls the connection of the generator D to the line wires B and C; these two wires being normally connected to the telephone battery E. The switch G controls the connection to the third wire A of the generator D, said line wire A being normally connected to one side of the battery F, which is grounded at f at its opposite side. The reversing key H provides means for connecting either line wire B or line wire C to the battery F. Gang key J at its one side controls the connection of the differential relay K to the line wires B and C, and at the other side controls the connection of said line wires to the battery E.

The various connections at the central station are in detail as follows: Points 1 and 4 of switch G are connected to opposite terminals of the generator D. Point 2 is connected to contact 28 of the gang switch J. Point 8 is connected to contacts 17 and 30. Contact 5 is connected to line wire B and contact 6 is connected to line wire C. Point 7 of the switch G is connected to one terminal of generator D and point 8 is connected to contact 27 of the gang key. Point 9 is a dead point. Point 10 is grounded at 5/. Contact 11 is connected to line wire A, and contact 12 is connected to the other pole of generator D. Point 13 of the reversing key H is connected to one pole of battery E. Points 14- a nd 16 are dead points and point 15 is connected to the battery E in multiple with point 13. Contact 17 is connected to contact 30 of the gang key and contact 18 is connected to the selector I and also to point 21 of gang key J. Point 19 of the gang key is connected to the signal battery F and also to ground f. Point 20 is con nected to the other pole of battery F. Point 21 is connected to selector I and point 22 is connected to the other side of the selector. Points 28 and 26 are dead points. Points 24; and 25 are connected to opposite ends of the diflerential relay K. The contact 27 is connected to contact 8 of switch G, and the contact 28 is connected to contact 2 of the switch G. The contact 29 is connected to, but insulated from the contact 27, and is arranged to be moved into connection with point 19 by the operation of the gang key.

The contact 30 is connected to contact li of the reversing key.

The differential relay K has two opposed balanced windings, as shown, with a ground connection is at the junction of the two windings, and this relay is, as before stated, normally connected across the line wires B and C. A non-inductive resistance K is connected in parallel with the windings of the relay K to carry the load in using the metallic circuit in telephoning. By the provision of these coils and the condensers S and S at the end of the line, there is provided a perfectly balanced circuit for telephone purposes. The foregoing connections are such that the wires B and C are normally connected to battery, so that any box or call station can call up and talk with the central station. The relay K is also connected in circuit in such a manner that, as will hereinafter be more fully described, it will operate the armature contact 7a to connect in the register or recorder L, to record any message which may be sent in at any time from the code transmitter P of any call station.

The normal telephone circuit from any call station may be traced as follows: from the transmitter to line wire C; to contact 6; contact point 3 ;'contact 30, point 2 1 to one terminal of relay coils K, through said coils to contact 29, to battery E, to contact point 15, to contact 18, to contact point 21, to contact 28, to point 2, to contact 5, to line wire B, and back to the transmitter. The resistance K in parallel with the relay coil K will, however, take the load current, and practically no current passes through the coils K. \Vhen the transmitter 41 at the call box is operated the wire B is grounded at 43, through the contact point 3a, the contact 36, and the contacts a1 and 12. Current now flows from the battery E in one direction to contact 29, point 24 to and through one-half of the winding K to ground at and from the opposite pole of the battery to contact point 15, contact 18, to contact point 21, to cont-act 28, to point 2, to contact 5 to line B and thence to ground at 4-3, through the contact point 34, contact 36, and the contacts 41 and 42. The line wire A is also normally connected to the normally grounded signal battery, so that the grounded lamps Q, at each call station can be lighted, whenever the door of a call box is opened, the opening of this door controlling a contact g which controls this ground connection. Wire A is also grounded at the distant end of the system.

The operators telephone switch OS has its movable contact 8 connected in gang with the movable contact of the generator switch G, as indicated in Fig. 2, in such a manner that when the generator switch is operated, the operators telephone will be out out and vice versa. The primary circuit of the telephone transformer is also shown as connected to the line wires B and 0 through the contact 30, of the gang key J, so that the operators telephone will be cut out when the gang key is operated. In this manner all interference between the box calling and box telephoning operations is avoided.

The apparatus and circuit arrangement at each call station is as follows: The telephone switch O (which may be of the usual plug type) has the four contact points 31, 32, 33 and 34, and the two movable contacts 35 and 36. Points 31 and 33 are dead points. Point 32 is connected to the armature of the transmitter of the telephone 0. Point 34 is connected through a retardation coil 38 to the line wire B. The line wire C is connected through a condenser 39 to one end of a coil 40, of the relay R. The other terminal of this coil is connected to the line wire B, and the coil also has an intermediate connection to the transmitter 45 of the telephone 0. The contact 35 is connected to the line wire C and the contact 36 is connected to the movable contact 41 of the code transmitter P, having the fixed contact 42 grounded at 43. 44 is the telephone receiver connected to the secondary of the induction coil. The transmitter 45 is connected at one side to the primary of the induction coil and at the other side to an intermediate point of the coil 40, as before described. The relay R has a pickup coil 47, one terminal of which is grounded at 48, and the other terminal of which is connected through a condenser 49 with the contact 50 of the selector N. 51 and 52 are the pick-up and restoring armatures, respectively, of the relay R. The lamp Q is connected at one ter minal to line wire A and by its other terminal to the movable contact 53 of the selector N. This lamp is also connected to ground through a normally open contact 54, controlled by the armature 51. The selector N has two coils, viz., a step-up coil 55 which is in series in the line wire B, and a release coil 56 which is in series in the line wire 0. Each of these coils has a non-inductive shunt 57, which carries the normal telephone load current. The coils 55 of all the call boxes or stations are in series with each other in the line B, and the coils 56 of all the call boxes or stations are in series with each other in the line C. Each coil has a metallic core 58, arranged when the coil is energized, to attract an armature 59, carrying a pawl 60, arranged to engage a ratchet 61, on a shaft 62 of the selector. The armature is arranged to be retracted after each impulse by a spring 63.

On the shaft 62 is shown a selector wheel 64 and also a contact wheel 65, having a contact projection 66, which is arranged to engage the movable contact 53, before described, and close it against the fixed contact 50. When the projection 66 causes the engagement of the contacts 50 and 53, it closes the circuit of the coil 47 from the line wire A to ground. The ratchet wheel 61 is provided with a holding pawl 67, which is arranged to be released by an armature 68, which is attracted by the energization of the coil 56. The shaft 62 has a helical spring 62, arranged to be wound as the shaft is revolved, and which acts to return the shaft'and contact wheel to their initial positions as soon as the holding pawl is released. Each of the contact wheels 65 is also provided with another contact projection 66 which is also adapted to close the contact 53 against the contact 50. 66 is a stop for the wheel 65 and which is arranged to engage a fixed stop 66.

The contact wheels 65 of the difierent call boxes or call stations are all set initially in different positions, as indicated in Fig. 1, so that they will be brought respectively to circuit-closing positions by a diiferent number of current impulses and actuations of the pawl 60 and ratchet 61, and thus enable a selection to be made. The contact projections 66 of these wheels are all set in corresponding positions, so that they will all come to circuit-closing position at the same time, and thus enable all the stations to be simultaneously called. Each of the coils 40 has a compoundwinding, one part being of relatively high resistance and the other part of relatively low resistance. The entire coil is in series across the line wires B and C, but is normally opencircuited to direct current by the condenser 39. The portion of lower resistance is in series between the line wires B and C, through the primary of the box telephone, and will be energized when the telephone is cut in. Each of the relays U and U consist of a winding 70, wound on a magnetic core and connected in the line wires B and C, and another winding 71 which is in the line wire A. The windings 70 when energized attract the movable contacts 73, to close the normally open shunt ground connections T T, and the windings 71 when energized act as restoring windings for said contacts. By sending current from the generator D over the wires B and C, windings 70 are energized to close the contacts 73, and by sending current from the generator D over the wire A, the windings 71 are energized to restore said contacts. It will be understood, however, that the windings T0 and 71 might both be connected in the wires B and O and be respectively responsive to different currents transmitted over said wires. By reason of the condensers S and S and their normally open grounded shunts, battery current will not pass beyond the last call station, but when the grounded shunts are closed, either the line B or the line C may be used as a separate grounded circuit. The coils 70, together with the condensers in series therewith, afford a perfectly balanced telephone circuit over the wires B and C.

It the operator at central station desires to call any one of the boxes selectively, the operation is as follows: The gang key J is first thrown to cut the differential relay K oil from the two-wire circuit B C, and at the same time to ground one side of the battery E through the contact 19; and this key is left in this position. This same movement of the gang key through the contact connects the selector to the line wire B. The switch G is then operated to send alternating current over the line wires B and C, and thus drop the contacts 78 at the far end of the line, and thereby ground both said wires. The arm of the selector I is then moved on its dial to the number corresponding to the number of the particular box being called, and its return movement sends out on the line B the required number of impulses to energize the coil 55 of the selector N, to move this selector into position to cause its contact 66 to close the contact 53 against the contact 50. It will be understood that the contact wheels of the entire series of selectors will be given a similar number of impulses, but that the contact wheel at the selected box is the only one which will be left in circuit-closing position. The operator now sends generator current out on the line wire A, to energize the coil 17, and thereby actuate the armature 51 to ground the lamp (Q, which is then lighted by the battery current flowing on said wire A, the gang J being now released to connect the battery to said wire. This also energizes the restoring windings 71 of the relays U U, and thus restores the normally open ground for the wires B and C. The release of the gang key also connects the battery across the line wires B and C with relay K in series. The reversing switch H is then moved to throw battery on to the line wire C, and thus energize the coils 56 of all the selectors to release their holding pawls and return the contact wheels to normal positions ready for another call. As soon as the person called goes to the box and cuts in the box telephone, a portion of the coil 40 is energized to restore the relay R, and open the grounding contact at 54, and thus extinguish the lamp.

VVhen it is desired to send in a code signal from any station, the box door is opened, thereby lighting the lamp Q; and the code transn'iitter P is actuated, according to the code wheel dial, to give the required number of impulses. At each impulse, the line wire 13 will be grounded at 43, through the closing of the contact at 42. The impulses will pass through the retardation coil 38, the line wire B, contact 5, the point 2, contact 28, point 21, contact 18, to battery E, to contact 29, point 2a and to and through a portion of the differential relay K and to ground at as before described. This energizes the relay K to attract the armature 7c and close the circuit of the register or recorder L. It will be readily seen that normally the relay K will not be energized by the normal battery current in the line wires 13 and C, since the two windings of its coil will oppose each other. hen, however, the ground connection at 43 is made, a circuit will be eliected through one of the windings of this coil to ground at 70. By means of the retardation coil 38 and the grounded circuit established when the code transmitter is operated, the operation of this transmitter will in no way interfere with the telephone circuit over the line wires B and C, and sending a code signal from one box will not interfere in the slightest with the use of the line for tele-l phoning from another box. If the operator has made a mistake in selecting the box, or if after a call has been sent, it is found to be unnecessary, and it is desired to restore the call box to its normal position so as to receive another call, the operator sends current from the generator D out on the line wires B and C, thereby energizing the coil 40 to restore the armature contact 52 and break the lamp circuit and extinguish the lamp. It the operator desires to call all the boxes simultaneously, a single impulse imparted by the operator to the line wire B will bring all the contacts 66 to their circuit-closing positions, and all the lamps Q will be simultaneously lighted.

7 1 designates a switch by means of which either pole of the generator D may be grounded when its other pole is connected to either line wire B or C, and whereby either of said wires may be grounded at the far end independently of the other and used as a grounded circuit.

My invention provides a very complete system by which a large number of boxes or call stations may be operated from a three-wire circuit common to all the stations, without interference, and with great facility of selection. It will be understood that the contact wheels 65 can either be made of suflicient size to permit of a large variation in the initial position of their con-" to still further increase the capacity of the system.

IVhile I have shown the call signal of each station as consisting of a lamp, it will, of course, be understood that any other suitable call signal, either visible or audible, or a combination of the two may be employed. It will also be understood that signaling instruments other than the particular ones described may be used at the boxes. Also that the switch devices for effecting the various circuit connections may be of any suitable character.

The use of a normal two-wire metallic circuit, with means whereby each of the wires may be grounded and used as a separate grounded circuit, constitutes a basic feature of the invention which is capable of wide use. This feature may, for instance, be availed of for telephone work, since it gives not only a complete metallic circuit between the connected points, but also makes available two separate grounded circuits between such points. While I have shown the restoring means for the distant end of these wires as being operated by current over the third wire, this is by no means essential, since, by using selective windings for the relays, both the pick-up and restoring coils of these relays can be energized by current sent over the two wires B and C.

I claim:

1. In a signaling system, a two-wire metallic circuit, the wires of which have a normally open ground connection at the distant end of the system, means for normally supplying current to said wires, and means for abnormally energizing said wires to close the open ground connections thereof, whereby they may be used as separate grounded circuits, substantially as described.

2. In a signaling system, a two-wire metallic circuit having normally open ground connections at the distant end of the system, means for temporarily energizing said Wires to close the ground connections, means whereby either one of the wires when so grounded may also be grounded through a source of current at the near end of the system and used as a grounded circuit, and means for subsequently reestablishing the open ground connections at the distant end of the circuit, substantially as described.

3. In a signaling system, a two-wire metallic circuit, means for normally energizing said circuit, the circuit wires having ground connections at the distant end of the circuit which are open-circuited with respect to the normal energizing current, and means for closing the ground connections to permit either one of said wires to be used as a separate grounded circuit, with other means for grounding the near end of each wire through a source of current, substantially as described.

I. In a signaling system, a two-wire metallic circuit having normally open ground connections at the distant end of the system, means for closing the ground connections, a plurality of call boxes or stations connected .in multiple across said circuit, a third wire also grounded at the distant end,-a signal at" each station connected to the third wire and having a normally open ground connect1on,a circult-closlng device controlling the ground connection with such signal, and means for energizing the third wire to close said ground connection, substantially as described.

5. In a signaling system, a two-wire metallic circuit, a plurality of call stations or boxes having signal means connected in parallel across said circuit, the wires of said circuit being grounded at the distant end of the system by connections which are normally open with respect to the normal current flowing in said wires, means whereby the wires may be energized to close the ground connections, means for grounding the near ends of either of said wires through a source, and other means for restoring the normally open ground connection at the distant end, substantially as described.

6. In a signaling system, a central station, a plurality of call stations, a three-wire circuit leading out from the central station and common to all the call stations, two of'the wires forming a normal metallic circuit through which communication may be had between the central station and any of the call stations and having normally open ground connections at their distant ends, means for closing the ground connections and for using each of said wires as a separate grounded circuit, signal mechanism at each call station connected to the third wire, and means whereby the circuits of said signal devices may be controlled by the twowire circuit, substantially as described.

7. In a signaling system, a central station,

a plurality of call stations, three line wires leading out from the central station and common to all the call stations, two of said wires forming a normally closed metallic circuit with open ground connections at the far end of the system, a signal device at each of the call stations connected to the third wire and having a normally open circuit, and selective relays at the call stations having windings included in the wires of the two-wire metallic circuit and arranged to control the circuits of the signal devices,

together with means whereby each of the wires of the metallic circuit may be used as a separate grounded circuit, substantially as described.

8. In a signaling system, a central station, a plurality of call stations, three circuit wires leading out from the central station and common to all the call stations, two of said wires having normally open ground connections at the far end of the system and forming a metallic circuit, signaling instru ments at the call stations connected in parallel across these two wires, a signal device at each call station connected to the third wire and having a normally open circuit, and selectors at the call stations having a winding in series in each of the wires of the two-wire circuit and arranged to control the circuits of the signal devices connected to the third wire, substantially as described.

9. In a signaling system having a central station, a plurality of call stations and three line wires leading out from the central station and common to all the call stations, two of said wires constituting a normal metallic circuit with open ground connections at the far end of the system, and the third wire being grounded through a source of current at central station and also grounded at the far end of the system, signaling mechanism at each call station connected across the wires of the metallic circuit, a signaling d evice at each station connected to the third wire, selectors at the call stations arranged to control. the circuit of said signaling devices and having a winding in series in each of the wires of the metallic circuit, and means whereby each of the wires of the metallic circuit may be separately grounded at each end and used as a separate grounded circuit, substantially as described.

10. In a signaling system, a central station, a plurality of call stat-ions, three line wiresleading out from the central station and common to all the call stations, two of said wires constituting anormal two-wire metallic circuit, and the third wire being normally grounded at each end, means whereby either of the first named wires may be separately grounded at its ends and used as a grounded circuit, and signaling mechanism connected to said wires, substantially as described.

11. In a signaling system, a central station, a plurality of call stations, three line wires leading out from the central station and common to all the call stations, two of said wires forming a normal two-wire metallic circuit with open ground connections at the far end of the system, and the third wire being grounded at the far end of the system and normally grounded through a source at the central station, signal mechanism at the several stations connected in parallel across the wires of the two-wire circuit with signaling mechanism at 'each call station connected at one side to the third wire and having a normally open ground connection, selectors at the call stations having a winding in each wire of the two-wire circuit and arranged to control the ground connections of the signal devices, and means for separately grounding each of the wires of the two-wire circuit and for using the same as a grounded circuit, substantially as described.

12. A signaling system, comprising a central station, a plurality of call stations, three line wires leading out from the central station and common to all the call stations, two of said wires constituting a normal twowire metallic circuit, each of said wires being grounded through a condenser at the far end of the system and having also a normally open shunt ground connection, and a relay for controlling each of the shunt ground connections and having one winding in series in each line wire, the third wire being normally grounded through a source at the central station and also grounded at the far end of the system, and each of said relays having another coil in series in the third wire, together with signaling mechanism operatively connected to said wires, substantially as described.

13. In a signaling system, a central station, a plurality of call stations, three line wires leading out from the central station and common to all the call stations, two of said wires constituting a normal metallic cir cuit from which communication may be held between the central station and any call station, a signal device at each call station connected at one side to the third conductor and at the opposite side having a normally open ground connection, said third wire being normally grounded at each end, and a selector at each call station having one winding in series in one wire of the two-wire circuit and another winding in series in the other wire of the two-wire circuit and arranged to control the circuit of the signal device, said selectors having means for both selective and collective operation, substantially as described.

14. In a signal system, a central station, a plurality of call stations, a twowire metallic circuit leading out from the central station and common to all the call stations, telephone sets at the central station and call station connected in parallel across said wires, a code transmitter at each call station connected at one side to one of said wires and at the other side to ground, and a differential relay having opposing windings connected across said circuit at central station and arranged to control a local circuit, said relay having a ground connection, substantially as described.

15. In a signaling system, a central. station, a plurality of call stations, a two-wire circuit leading out from the central station and common to all the call stations, said circuit having each of its wires grounded at the far end of the system through a con denser, and also having a normally open shunt connection to ground, relays controlling the shunt ground connections, means for sending an alternating current from said line wires to energize the coils to close the shunt ground connections, and means t'or reversely energizing the relays to open the shunt ground connections, substantially as described.

16. In a signaling system, a central station, a plurality of call stations, a two-wire metallic circuit leading out from the cenral station and common to all the call stations, said circuit having its wires grounded at the far end of the system through condensers and also having normally open shunt ground connections, relays for con trolling the shunt ground connections, means for normally energizing said wires with direct current, means for temporarily energizing said wires with alternating current to energize the relays to close the shunt ground Connections, and other means for energizing the relays to open the shunt ground connections, substantially as described.

17. In a signaling system, a central station, a plurality of call stations, a two-wire metallic circuit leading out from the cen tral station and common-to all the call stations ,said circuit having its wires grounded at the far end of the system through condensers and also having normally open shunt ground connections, relays for controlling the shunt ground connections, means for normally energizing said wires with direct current, means for temporarily energizing said wires with alternating current to energize the relays to close the shunt ground connections, and other means for energizing the relays to open the shunt ground connections, together with means for grounding each of said wires at the central station through a source of current to enable said wires to be used as a grounded circuit, substantially as described.

18. In a signaling system, a central station, a plurality of call stations, a two-wire metallic circuit leading out from the central station and common to all the call stations, telephone instruments at each call station and at the central station connected in parallel across said circuit, a difierential relay at the central station having opposed balanced windings normally connected across said circuit, a local circuit having a contact device to be controlled by said relay, and signaling instruments at the call stations connected to one wire of the said circuit and to ground in parallel, the differential relay also having a ground connection at the junction of its windings, substantially as described.

19. In a signaling system, a central station, a plurality of call stations, a two-wire metallic circuit leading out from the central station, means for normally energizing said circuit, the wires of said circuit having normally open ground connections with respect to the normal current flowing therein, signaling instruments at the several stations connected in parallel across said circuit, means for separately grounding each wire of said circuit at the far end of the system and for using each of said wires as a separate grounded circuit, a third wire also leading out from central station and normally grounded at both ends, a signal device at each call station connected with the third wire and having a normally open ground connection, a selector at each call station having one winding in each wire of the two-wire circuit and arranged to partially control the circuit of the signal device,

and a relay at each call station having a coil in series with the selector-controlled circuit and further controlling the circuit of the signaling device, substantially as described.

20. In a signaling system, a central station, a plurality of call stations, a two-wire metallic circuit leading out from the central station, means for normally energizing said circuit, the wires of said circuit having a normally open ground connection with respect to the normal current flowing therein, signaling instruments at the several stations connected in parallel across said circuit, means for separately grounding each wire of said circuit at the far end of the system and for using each of said wires as a separate grounded circuit, a third wire also leading out from central station and normally grounded at both ends, a signal device at each call station connected with the third wire and having a normal open ground connection, a selector at each call station having one winding in each wire of the two-wire circuit and arranged to partially control the circuit of the signal device, and a relay at each call station having a coil in series with the selector-controlled circuit and further controlling the circuit of the signaling device, said relay having another winding connected across the two- Wire circuit, substantially as described.

21. In a signaling system, a central station, a plurality of call stations, a two-wire metallic circuit leading out from the central station, means for normally'energizing said circuit, the Wires of said circuit having a normally open ground connection with respect to the normal current flowing there in, signaling instruments at the several stations connected in parallel across said circuit, means for separately grounding each wire of said circuit at the far end of the system and for using each of said wires as a separate grounded circuit, a third wire also leading out from the central station and normally grounded at both ends, a signal device at each call station connected with the third wire and having a normal open ground connection, a selector at each call station having one winding in each wire of the two-wire circuit and arranged to partially control the circuit o1 the signal de vice, and arelay at each call station having a coil in series with the selector-controlled circuit and further controlling the circuit of the signaling device, said relay having another winding connected across the two wire circuit, the last named winding having an intermediate connection through another circuit-closing device at the call station, substantially as described.

22. In a signaling system, a central station, a plurality ot call stations, a two-wire metallic circuit leading out from the central station and common to all the call stations, telephone instruments at the several stations connected in parallel across the circuit, a differential relay at the central station connected. across the circuit and having opposed balanced windings, and a central ground connection, a local circuit contain.- ing a recorder arranged to be controlled by the di'll'erential relay, and a signal transmitting device at each call station connected at one side to one wire of said circuit through a retardation coil and having a normally open ground connection at its opposite side, substantially as described.

23. In a signal system, a central station, a two-wire metallic circuit leading out from the central station, signaling instruments at each call station connected across said circuit, said wires having ground connections at the far end of the system which are open with respect to the normal current flowing in said wires, a third wire also leading out from the central station and common to all the stations, said wire being normally grounded at both ends, a source o't current normally connected across the twowire circuit at the central station, a different current source normally disconnected from the two-wire circuit, means for temporarily cutting olt the first named source of current and connecting the second named source of current to said wires, a circuit-closing device arranged to be energized by current from the second source to close the ground connection, another source of current at central station grounded at one side with means whereby it may be connected to either of the two wires when grounded, a signaling device at each call station connected to the third wire and also having a normally open ground connection, selectors at the call stations having a winding in each of the wires of the two-wire circuit and controlling the circuit of the signal devices, said relays having coils in series with the third wire, and means for energizing the third wire with either one of two ditl'erent kinds of current, substantially as described.

24;. In a signaling system, a central station, a plurality of call stations, three line wires extending out from the central station and common to all of the call stations, a selector at each call station having one winding in series in one of said wires and having another winding in series in another of said wires, said windings being arranged to actuate the selector in opposite directions, means for separately energizing the wires containing these coils, and a signaling device at each call station connected with the third wire and having a circuit arranged to be controlled in part by the selector, substantially as described.

25. In a selective signaling system, a metallic two-wire circuit common to a plurality of stations and normally available for communication between the said stations and a central station, a third wire also common to all the stations and grounded. at its ends, a call signal at each station, said signals being connected on open circuits and in parallel between the third conductor and ground, and a selector at each station controlling the ground connection of the call signal and having an actuating coil in one wire of the two-wire circuit and a restoring coil in the other wire of the two-wire circuit, together with means whereby either of said two wires may be separately energized, substantially as described.

26. In a selective signaling system, a metallic two-wire circuit common to a plurality of stations and normally available for communication between the said stations and a central station, a third wire also common to all the stations and grounded at its ends, a call signal at each station, said signals being connected on open circuits and in parallel between the third conductor and ground, and a selector at each station controlling the ground connection of the call signal and having an actuating coil. in one wire of the two-wire circuit and a restoring coil in the other wire of the two-wire circuit, together with means whereby either of said two wires may be separately energized, and means at each call station for re-opening the ground connection of the call signal, substantially as described.

27. In a selected signaling system, a metallic two-wire circuit common to a plurality of stations and normally available for con'imunication between the said stations and a central station, a third wire also common to all the stations and grounded at its ends, a call signal at each station, said signals being connected on open circuit and in parallel between the third conductor and ground, and a selector at each station controlling the ground connection of the call signal and having an actuating coil in one wire of the two-wire circuit and a restoring coil in the other wire of the two-wire circuit, together with means whereby either of said two wires may be separately energized, and a relay at each call station having a winding connected across the two-wire circuit and arranged to re-open the ground connection of the call signal, substantially as described.

28. In a selective signaling system, a metallic two-wire circuit common to a plurality of stations and normally available for communication between the said stations and a central station, a third wire also common to all the stations and grounded at its ends, a call signal at each station, said signals being connected on open circuit and in parallel between the third conductor and ground, and a selector at each station controlling the ground connection of the call signal and having an actuating coil in one wire of the two-wire circuit and a restoring coil in the other wire of the two-wire circuit, together with means whereby either of said two wires may be separately energized, and a relay at each call station having a pick-up winding controlled by the selector, and a restoring winding connected across the two- Wire circuit, substantially as described.

29. In a selective signaling system, a metallic two-wire circuit common to a plurality of stations and normally available for communication between the said stations and a central station, a third wire also common to all the stations and grounded at its ends, a call signal at each station, said signal being connected on open circuit and in parallel between the third conductor and groimd, and a selector at each station controlling the ground connection of the call signal and having an actuating coil in one wire of the two-wire circuit and a restoring coil in the other wire of the two-wire circuit, together with means whereby either of said two wires may be separately energized, a relay at each call station having a pick-up winding controlled by the selector and a restoring winding connection across the trolled by the selectors, substantially as described. I

31. In a signal system, the combination with two-circuit wires, of a plurality of selectors, each having an actuating winding in series in one of said wires and a restoring coil in series in the other of said wires, means for separately energizing each of said wires, and signal devices having circuits controlled by the selectors, the selectors having contacts which close the circuits simultaneously throughout the series, and other contacts which act selectively, substantially as described.

32. A signaling system comprising a twowire circuit normally available for communication between a central station and a plurality of call stations, the wires of said circuit having normally open ground connections at the far end of the system, relay devices controlling said ground connections, means at the central station for operating said relays, other means at central station for grounding the central station end of either of said wires and for energizing the grounded wires, and signal mechanism connected to said wires to be operated therefrom when so grounded, substantially as described.

In testimony whereof I have hereunto set my hand.

ALFRED K. DEMENT.

Witnesses GEO. B. BLEMING, GEO. I-I. PARMELEE.

Copies of this patent may be obtained for five cents each, by addressing the Commissioner of Patents,

Washington, D. G. 

